The present invention relates to medical instruments and, more particularly, to a system and method for detecting fractionated cardiac potentials. A major objective of the present invention is reliable characterization of fractionated cardiac potentials which can be a prodrome of sudden cardiac death.
Sudden cardiac death (SCD) is usually the result of primary electrical instability of the heart. This instability can result when compromised regions of a heart disrupt the electrical signal which activates the heartbeat. These disruptions, while not always fatal, cause fractionated potentials, which provide clues as to a subject's susceptibility to potentially fatal heart problems.
A fundamental challenge in cardiology is to obtain electrocardiograms which are sufficiently free of noise that fractionated potentials can be detected reliably. Being very weak signals, fractionated potentials occurring during electrically active periods of the hearbeat are readily masked by the much stronger cardiac signals. In addition, even when fractionated potentials occur during periods otherwise free of cardiac signals, they can be masked by non-cardiac electrical activity. If the electrical potentials associated with heartbeat activation are precisely registered and carefully processed, information about the fractionated potentials can be recovered reliably. Less careful acquisition and processing can allow the miniscule fractionated potentials to be missed.
Electrocardiograms are usually obtained non-invasively to minimize subject stress and inconvenience, as well as medical expense. Typically, cardiac electrical potentials are detected by electrodes on the body surface connected to an electrocardiograph (ECG) which plots the various potentials as a heartbeat waveform. The cardiac potentials at the electrodes are quite weak, in part, because of current-shunting by tissue between the heart and the skin surface.
The weakness of the monitored cardiac signals makes them vulnerable to noise. The noise can arise from stray electrical fields external to the subject and from non-cardiac fields within the subject. Most notably, electrical potentials from the subject's muscular activity and tonicity are superimposed on the cardiac potentials. This superposition makes it more difficult to obtain the accurate heartbeat waveforms required for reliable detection of fractionated potentials.
From an information theory perspective, the problem of accurate representations of electrocardiac activity corresponds to a need to improve the signal-to-noise ratio for the cardiac potentials. Time-averaging is a well-known approach to improving the signal-to-noise ratio for a periodic waveform. When several successive single-cycle waveform segments are aligned and superimposed, the common components reinforce each other while random noise components tend to cancel. This approach has been used often in the detection of abnormal heartbeat patterns (arrhythmias). The arrhythmias can be detected by monitoring a subject over a long interval, making use of a compact, ambulatory ECG such as a Holter recorder.
The fractionated potentials associated with SCD are weaker than the potentials associated with arrhythmias detectable in Holter recordings, and additionally, they can fluctuate. Electrical signals that are shunted around abnormal regions of heart tissue along various routes generate variable electrical potentials at the skin surface. To the extent that the resulting fractionated potentials vary from heartbeat to heartbeat, they will be cancelled rather than enhanced by time-averaging. The achievement of signal-to-noise enhancement by time averaging when prospecting for fractionated potentials is debilitated by the irregularity of the fractionated potentials. In this circumstance, both noise and signal are reduced by time averaging.
Another approach to improving the signal-to-noise ratio of the ECG signals is to minimize the sources of noise. Relaxation techniques and shallow breathing techniques have been used to reduce muscular activity and thus the amount of noise contributed by electrical activity in the muscles. However, the noise reduction provided by these techniques is limited. Furthermore, these techniques require subject relaxation, which may not be achieved or sustained. The requirement of subject relaxation can limit the practical duration over which reliable monitoring can occur.
The duration over which monitoring can occur is important since fractionated potentials occur sporadically in some subjects. Thus, characterization of fractionated potentials can require monitoring a subject over many hours. The length of time required for monitoring a subject introduces another problem: the volume of electrical activity data generated by a subject over such an extended period can make it difficult to identify electrical patterns of interest. Thus, there is a need for some preselection of data to reduce the field of search for the cardiac events of interest.
What is needed is a system and method for detecting fractionated potentials so that a subject's risk of sudden cardiac death can be assessed with greater confidence than previous methods allow. This requires high-precision multichannel electrocardiograms obtained without using time-averaging techniques, which techniques can wash out irregular potentials. Provision for convenient monitoring of a subject over extended time periods is required to enhance the odds that fractionated potentials will be detected, yet the amount of data collected should be limited to an amount which can be practically examined for the fractionated potentials of interest.